This invention relates to a compressor surge detector for turbine type power plant and particularly to the means for opening the compressor bleeds at a rapid rate to eliminate surge in response to the pressure downstream of the compressor and for holding the bleeds in the open position until after surge subsides.
As is well known in the art, compressor surge for axial flow types of compressors utilized in turbine types of power plant has been a problem that has plagued the aircraft industry since its inception. Compressor surge is defined as that effect which results from stalling a sufficient number of compressor blades so that a momentary reversal of airflow occurs thru the compressor. This causes compressor discharge pressure to drop very rapidly and sometimes results in continued pressure oscillations until some action is taken to eliminate the surge conditions.
While the theory of surge is not readily understood suffice it to say that this rapid change in pressure and the ensuing pressure pulsation not only could interfere with the efficiency of the power plant but can also incur damage or disaster thereto. Thus it is important that means are incorporated to eliminate or prevent surge from occurring. One method of preventing surge from occurring is by opening up bleed valves prior to or at the onset of the oncoming of the surge condition. In the heretofore known open loop types of controls the control is designed to schedule the opening of the bleeds and/or control fuel flow so as to operate the power plant below the surge line which is well defined for each given engine.
It is also well known in the art to measure the compressor discharge pressure and actuate the bleed valves whenever a predetermined pressure change or rate of change occurs. The problem with the latter mentioned method is that when the pressure fluctuation is such to indicate a return to the normal condition the bleed valves are again returned closed, which may occur prior to the elimination of the surge condition. Also the condition may occur where the bleed valves are oscillating between open and closed during the surge condition.
I have found that I can obviate the problems noted above by sensing compressor discharge pressure and sizing a control system so that a predetermined, rapid decrease of sensed pressure will rapidly move a metering valve which ports flow to the bleed actuator so as to open the engine bleed ports. The metering valve is designed to overtravel its null position to supply high pressure to the bleed actuator to maintain the bleeds open.
By incorporating a combination of restrictor and check valve judiciously located and sized, the system will move the metering valve rapidly in a direction to open the engine bleed ports and will cause the metering valve to return to a position which calls for closed bleed ports at a much slower rate. Thus sizing th restrictor and controlling the amount of valve overtravel will determine the time delay before the actuator returns the bleeds to their normal closed position and hence allows the surge conditions to be eliminated before the bleeds are closed.